1. Field of the Invention
The present invention relates to a radial piston engine with roller guides for axial guidance of the rollers, via which the pistons are braced against the cam ring.
2. Discussion of the Background
From British Patent GB B 2238086 there is known a radial piston engine corresponding to the preamble of claim 1, wherein the object was to reduce the manufacturing complexity previously associated with axial positioning of the rollers and thus to lower the manufacturing and assembly costs. This object was supposedly achieved in that, in each of the spaces between the roller end faces and the cylindrical inside face as the roller guide, there is disposed a wedge piece, whose cross section viewed in piston displacement direction is a circular segment, and which positions the roller axially in the cylinder relative to the cam. ring. The wedge pieces disclosed in the cited patent can be divided substantially into three different designs.
In a first design, each wedge piece has on the side facing the roller a plane surface which bears on the end face of the roller and on the side facing away from the roller a cylindrical surface which is in contact with the inside face of the cylinder. In addition, this wedge piece is accommodated in the space between roller, piston and cylindrical inside face, without being rigidly connected to any of these components. By virtue of the fact that the wedge piece is freely movable in piston displacement direction relative to the roller, the piston urges the wedge pieces toward the cam ring during a load stroke. In the process, the wedge pieces do not yet come into contact with the cams formed on the cam ring. In the ensuing idle stroke of the piston, however, the wedge pieces strike the cams formed on the cam ring. This recurring striking contact between the cams and the wedge pieces as well as the friction occurring therebetween can lead to severe wear of the wedge pieces. A further disadvantage of this design is that the width of the cam ring must be at least sufficiently large that the cam ring in addition to the roller also provides a contact face for the two laterally disposed wedge pieces. Aside from large total weight of the radial piston engine, a broad cam ring results in high manufacturing costs due to the greater manufacturing complexity associated with precision machining of the cam ring.
In a second design, each of the two wedge pieces has on the side facing the roller a driver-like projection, which extends toward the other wedge piece in an opening provided in the piston between piston and roller. Since in this case the wedge pieces are driven back into the cylinder by the roller during an idle stroke of the piston, striking contact between the wedge pieces and the cams basically cannot occur. Due to the high relative velocity between the roller and the driver-like projections of the wedge pieces in contact with the outside circumference of the roller, however, severe abrasion can take place on the driver-like projections. With increasing abrasion of the driver-like projections, the space available for play of the wedge pieces in piston displacement direction could ultimately increase to the point that striking contact could occur between the wedge pieces and the cams. A further disadvantage of this design is that driver-like projections must be formed on the roller guides and recesses must be formed on the piston, thus increasing the manufacturing complexity and thus the manufacturing costs.
In addition, the friction between the roller and the wedge pieces in the two foregoing designs occurs over the entire end face of the roller. This relatively large-area frictional contact necessitates precision machining of the friction faces of both components, once again resulting in high manufacturing costs.
At that time a further assumption was that, by virtue of the recurring contact of the rollers with the cam ring, the rollers and thus the respective piston automatically assume a particular angular position in the cylinder or relative to the cam ring, which is critical for reliable operation of the radial piston engine. In certain cases, however, for example during initial operation of the radial piston engine, when hydraulic fluid is fed to the cylinders for the first time, it is conceivable that the pistons and thus the rollers may be inserted so far into the respective piston that contact between roller and curved path does not yet occur. To ensure the necessary angular position of the piston in the cylinder in those cases, one of the two wedge pieces has, in a third design, on the side facing the cylindrical inside face, an elongated slot, into which a bolt, clamp or the like extending through the cylinder wall engages and in this way prevents turning of the piston in the cylinder and thus of the roller relative to the cam ring. The presence of the slot leads to weakening of the wedge piece in question, however, and thus to shortening of the useful life of the wedge piece, which is subjected to severe stresses and strains during operation of the radial piston engine. In addition, sliding contact takes place between the wedge piece and the bolt or clamp, which appears disadvantageous as regards good roller guidance. Furthermore, such a structural feature means high manufacturing complexity.
In view of the disadvantages encountered in conventional radial piston engines, the object of the present invention is therefore to provide an optimally engineered radial piston engine, which is characterized by greater manufacturing simplicity and at the same time functionally reliable operation.
This object is achieved by the inventive subject matter according to the features of claim 1, which is characterized in particular in that the roller guides disposed on the front sides of the rollers are rigidly connected to the respective roller with respect to sliding in piston displacement direction.
Since the respective roller urges the roller guides in piston displacement direction, in other words both during the load stroke and during the idle stroke, the roller guides can be dimensioned such that they do not project beyond the outside circumference of the respective roller, thus ensuring that striking contact does not take place between the roller guides and the cam ring. This ultimately leads to longer useful life of the roller guides and thus to functionally reliable operation of the radial piston engine on the whole. In addition, material economies are achieved in the manufacture of the roller guides.
If the roller guides do not come into contact with the cam ring, the width of the cam ring can be further reduced to a width which corresponds at most to the width of the rollers. In this way, not only is the total weight of the radial piston engine lessened, but also manufacturing complexity and thus the manufacturing costs are considerably reduced with regard to precision machining of the curved paths of the cam ring.
By suitable structural measures, such as by formation of a cylindrical projection on the roller guide, which projection is inserted in a corresponding recess on the front side of the roller, the relative velocities at the friction faces of roller and roller guides and thus the wear of the two components can also be reduced. Because of the reduced wear, the play developed between roller and roller guides is kept to a minimum, even after prolonged operating time. This contributes to functionally reliable and dependable operation of the radial piston engine on the whole.
Further advantageous features of the inventive radial piston engine are subject matter of the dependent claims.
The roller and roller guides can be rigidly connected with respect to sliding in displacement direction by simple manufacturing techniques. For example, the roller guides can be provided on the side facing the end face of the roller with a cylindrical projection, which is insertable into a central cylindrical recess formed on the end face of the roller. Likewise, it would naturally also be conceivable for the roller guides to be provided on the side facing the front side of the roller with a recess into which a projection formed on the front side of the roller is insertable. Of course, the opening and the projection engaged therewith could also have conical shape. If the diameter of the openings and projections are dimensioned such that they are small compared with the outside diameter of the roller, or in other words such that the projections and the openings are concentrated on a central region around the axis of rotation of the roller, and if the projections are engaged with the respective openings in such a way that play is present between the annular face of the roller around the opening and of the face around the projection of the roller guide, the circumferential velocities present at the outside circumference of the projection and at the inside circumferential wall of the opening during operation of the radial piston engine are reduced, as is therefore the relative velocity between roller and roller guides. Moreover, since no friction occurs at the two spaced-apart faces of roller and roller guide, reduction of frictional abrasion of both components is achieved.
The surfaces of the roller guides in contact with the cylindrical inside face preferably have cylindrical shape for simplicity, thus achieving optimal guidance of the piston-roller system in the cylinder. Since the roller is normally not subjected to compressive loads in axial direction, it would also be conceivable, however, to give the surface of the roller guide bearing on the cylindrical inside face rotationally symmetric shape or to construct it as a spherical segment relative to the axis of rotation of the roller. In this case, rigid connection between roller guide and roller would even be possible both in piston displacement direction and in the direction of rotation of the roller. This would have the advantage that, during operation of the radial piston engine, friction between roller and roller guides would no longer take place and the friction occurring between roller guide and cylindrical inside face would be considerably reduced.
Heretofore it has been assumed that the roller is constantly in contact with the curved paths of the cam ring, whereby the angular position of the piston in the cylinder and accordingly the orientation of the roller relative to the curved path is automatically predetermined. As already explained in the introduction, however, it is possible for the contact between roller and cam ring to be separated. In this case it is possible according to the present invention to prevent turning of the piston in the cylinder by means of an antirotation device disposed separately from the roller guide and thus to maintain a particular orientation of the roller relative to the cam ring.
For this purpose, the piston can be provided on the end portion facing away from the roller with a flattened part oriented perpendicular relative to the axis of rotation of the cylinder block, which flattened part bears on a corresponding contact face of the antirotation device, thus unambiguously predetermining the angular position of the piston in the cylinder and of the roller relative to the curved path. Another effective expedient has proved to be providing the cylinder with at least two cylinder portions of different inside diameters and the piston accordingly with at least two piston portions of different diameters matching the corresponding cylinder diameters. In this case, the antirotation device is provided in the cylinder portion with the smaller inside diameter and the flattened part is provided accordingly on the piston portion with the smaller diameter. By this expedient, on the one hand a large face for admission of pressure is retained on the piston and on the other hand only little material is removed from the piston in order to form the flattened part. The additional antirotation device creates better guidance of the piston in the cylinder compared with the conventional antirotation device mentioned in the introduction, since according to the present invention large-area sliding contact takes place between piston and antirotation device.
The antirotation device preferably has a cross section which, viewed in piston displacement direction, is a circular segment, with an arc corresponding to the cylindrical inside face and a chord corresponding to the flattened part.
Since the cylinder block in any case is normally provided with axial inlet ports, through which the hydraulic fluid enters the respective cylinder spaces, the manufacturing complexity with regard to fixing the antirotation device in the cylinder can be reduced in that the antirotation device is fixed in the cylinder by means of a pin, which is inserted through the inlet port into a blind hole in the cylinder block aligned with the inlet port.